Power Management System for a Piston-Based Implantable Infusion Pump

ABSTRACT

An implantable infusion device includes a pump, a charge storage unit, and a charging circuit configured to supply current to the charge storage unit from a battery in preparation for actuating the pump. The implantable infusion device also includes a pump actuator circuit configured to actuate the pump using energy from the charge storage unit, and a voltage boost circuit configured to provide a boosted battery voltage generated from the battery. The charging circuit is configured to supply current to the charge storage unit from the voltage boost circuit instead of directly from the battery in response to (i) a comparison of a voltage of the battery with a predetermined threshold and (ii) a comparison of a voltage of the charge storage unit with the voltage of the battery.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No. ______(Attorney Docket No. 5074N-000028), entitled “CHARGE PUMP SEQUENCING FORMINIMIZING IN-RUSH CURRENT,” which is being filed concurrently herewith.The disclosure of the above application is incorporated herein byreference in its entirety.

FIELD

The present disclosure relates to electrical systems for an implantabledrug pump and more specifically to power management systems for apiston-based implantable infusion pump.

BACKGROUND

Implantable drug pumps allow drug therapies to be administered to apatient outside of a clinical setting. Smaller amounts of the drug cantherefore be administered over long periods of time. In addition,implantable infusion pumps may inject drugs into areas such as theintrathecal space surrounding the spinal cord, where a drug is much moreeffective. For example, when administering a drug such as morphineintravenously instead of orally, only one-third of that drug may beneeded. In other words, the drug is three times as effective whenadministered intravenously. Morphine may generally be ten times aseffective when administered epidurally as compared to intravenously.Further, such a drug may be ten times as effective when beingadministered intrathecally as compared to epidurally.

Because an implantable drug pump is surgically implanted, the operatinglife of the implanted device needs to be as long as possible. Referringnow to FIG. 1, an example implantable infusion device 100 is shown. Thedevice 100 includes a reservoir 104 holding the drug to be administered.A pump 108 pumps drug from the reservoir 104 into the patient's body. Acontrol circuit 112 drives the pump 108 and is powered by a battery 116.

The operating life of the device 100 may be governed by factorsincluding the amount of drug stored in the reservoir 104 and the dosagerate, the life of the battery 116, and the mechanical longevity of thepump 108. In various implementations, the reservoir 104 may bereplenished with additional quantities of the drug while the device 100remains implanted in a patient. In such implementations, the battery 116is even more important in determining the lifetime of the device 100.Improving battery life may allow implanted devices to be replaced lessoften.

SUMMARY

An implantable infusion device includes a pump, a charge storage unit,and a charging circuit configured to supply current to the chargestorage unit from a battery in preparation for actuating the pump. Theimplantable infusion device also includes a pump actuator circuitconfigured to actuate the pump using energy from the charge storageunit, and a voltage boost circuit configured to provide a boostedbattery voltage generated from the battery. The charging circuit isconfigured to supply current to the charge storage unit from the voltageboost circuit instead of directly from the battery in response to (i) acomparison of a voltage of the battery with a predetermined thresholdand (ii) a comparison of a voltage of the charge storage unit with thevoltage of the battery.

In other features, the pump comprises a piston and a solenoid coil, andthe pump actuator circuit is configured to supply a current to thesolenoid coil from the charge storage unit. The predetermined thresholdis controlled in response to a firmware setting. The comparison of avoltage of the charge storage unit with the voltage of the batteryincludes comparing the voltage of the charge storage unit to apredetermined percentage of the voltage of the battery. Thepredetermined percentage is controlled in response to a firmwaresetting. A charge monitor circuit is configured to generate a signal inresponse to a comparison of the voltage of the charge storage unit witha predetermined level. The pump actuator circuit is enabled based on thesignal.

In further features, the charging circuit is configured to decrease thecurrent supplied to the charge storage unit in response to the voltageof the battery decreasing below a second predetermined threshold that isless than the predetermined threshold. In response to the voltage of thebattery decreasing below the second predetermined threshold, thecharging circuit is configured to halt supplying current to the chargestorage unit. After halting supplying current to the charge storageunit, the charging circuit is configured to begin supplying increasinglevels of current to the charge storage unit and verifying that thevoltage of the battery does not decrease below the second predeterminedthreshold.

In still other features, the implantable infusion device includes avoltage rail that is selectively connected to the battery. A holdmonitor circuit is configured to monitor a voltage of the voltage railand, in response to the voltage of the voltage rail decreasing below afirst threshold, activate the voltage boost circuit. An isolationcircuit isolates the voltage rail from the voltage of the battery inresponse to a difference between the voltage of the battery and thevoltage of the voltage rail. In response to the voltage of the voltagerail decreasing below the first threshold, the hold monitor circuitconnects the boosted battery voltage to the voltage rail. In response tothe voltage of the voltage rail increasing above a second threshold thatis greater than the first threshold, the hold monitor circuitdisconnects the boosted battery voltage from the voltage rail anddeactivates the voltage boost circuit.

In other features, a boost control circuit is configured to (i) controlthe voltage boost circuit and (ii) inhibit operation of the chargingcircuit while the voltage boost circuit is starting. The boost controlcircuit is configured to inhibit operation of an alarm circuit and atelemetry circuit while the voltage boost circuit is starting. Anelectronic port finder circuit is powered by the charging circuit. Thecharging circuit is configured such that supplying current to the chargestorage unit is mutually exclusive with powering the electronic portfinder circuit.

In further features, an alarm circuit is configured to generate anaudible alarm, and the alarm circuit is powered by the voltage boostcircuit. The alarm circuit disables the charging circuit while the alarmcircuit is generating the audible alarm. A telemetry circuitcommunicates with an external device. Operation of the telemetry circuitis inhibited while the voltage boost circuit is starting. The telemetrycircuit is configured to shut down in response to the voltage of thebattery decreasing below a second predetermined threshold that is lessthan the predetermined threshold.

A method of operating an implantable infusion device includes supplyingcurrent from a battery to a charge storage unit in preparation for acycle of a pump, and selectively actuating the pump using energy fromthe charge storage unit. The method includes performing a firstcomparison of a voltage of the battery with a predetermined thresholdand performing a second comparison of a voltage of the charge storageunit with the voltage of the battery. The method includes determiningwhether a boosted battery voltage is required in response to the firstcomparison and the second comparison, and when the boosted batteryvoltage is required, generating the boosted battery voltage andsupplying current to the charge storage unit from the boosted batteryvoltage instead of directly from the battery.

In other features, the pump comprises a piston and a solenoid coil, andthe method further comprising supplying a current to the solenoid coilfrom the charge storage unit to actuate the pump. The predeterminedthreshold is controlled in response to a firmware setting. The secondcomparison includes comparing the voltage of the charge storage unit toa predetermined percentage of the voltage of the battery. Thepredetermined percentage is controlled in response to a firmwaresetting. The method also includes generating a signal in response to acomparison of the voltage of the charge storage unit with apredetermined level, and selectively actuating the pump in response tothe signal.

In further features, the method includes decreasing the current suppliedto the charge storage unit in response to the voltage of the batterydecreasing below a second predetermined threshold that is less than thepredetermined threshold. The method also includes, in response to thevoltage of the battery decreasing below the second predeterminedthreshold, decreasing the current to the charge storage unit to zero.The method also includes incrementally supplying greater levels ofcurrent to the charge storage unit subsequent to decreasing the currentto zero; and verifying that the voltage of the battery does not decreasebelow the second predetermined threshold while supplying the greaterlevels of current.

In still other features, the method includes selectively connecting avoltage rail to the battery, monitoring a voltage of the voltage rail,and generating the boosted battery voltage in response to the voltage ofthe voltage rail decreasing below a first threshold. The method alsoincludes isolating the voltage rail from the voltage of the battery inresponse to a difference between the voltage of the battery and thevoltage of the voltage rail. The method also includes, in response tothe voltage of the voltage rail decreasing below the first threshold,connecting the boosted battery voltage to the voltage rail. The methodalso includes, in response to the voltage of the voltage rail increasingabove a second threshold that is greater than the first threshold,disconnecting the boosted battery voltage from the voltage rail andhalting generation of the boosted battery voltage.

In other features, the method also includes inhibiting the supplying ofcurrent to the charge storage unit while generation of the boostedbattery voltage is beginning. The method also includes operating anelectronic port finder function mutually exclusively with supplyingcurrent to the charge storage unit. The method also includes generatingan audible alarm powered by the boosted battery voltage. The method alsoincludes inhibiting generation of the audible alarm while generation ofthe boosted battery voltage is beginning. The method also includesinhibiting the supplying of current to the charge storage unit while theaudible alarm is being generated. The method also includes inhibitingcommunication with an external device while generation of the boostedbattery voltage is beginning. The method also includes shutting downcommunication with the external device in response to the voltage of thebattery decreasing below a second predetermined threshold that is lessthan the predetermined threshold.

Further areas of applicability of the present disclosure will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples areintended for purposes of illustration only and are not intended to limitthe scope of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a functional block diagram of an implantable infusion deviceaccording to the prior art;

FIG. 2 is a functional block diagram of an example implantable infusiondevice according to the principles of the present disclosure;

FIG. 3 is a schematic of an example implementation of a protectioncircuit;

FIG. 4 is a schematic of an example equivalent circuit of a battery;

FIG. 5 is a schematic of an example implementation of a multiplexercircuit;

FIG. 6 is a schematic of an example implementation of a isolationcircuit;

FIG. 7 is a schematic of an example implementation of a rechargecircuit;

FIG. 8 is a block diagram of an example implementation of a batterymonitor circuit;

FIG. 9 is a schematic of an example implementation of a thresholdcircuit;

FIG. 10 is a schematic of an example implementation of a variableresistor;

FIG. 11 is a block diagram of an example implementation of a chargemonitor circuit;

FIG. 12 is a block diagram of an example implementation of a CPU monitorcircuit;

FIGS. 13A-13B are block diagrams of example implementations of a holdmonitor circuit;

FIG. 14 is a schematic of an example implementation of a chargingcircuit;

FIG. 15 is a block diagram of example digital circuitry;

FIG. 16 is a functional block diagram of example functions of aprocessor;

FIG. 17 is a functional block diagram of an example implementation of apump control circuit;

FIG. 18 shows example traces of control signals that allow forelectronic port finder interleaving;

FIG. 19 is a flowchart depicting example operation of voltage boostcontrol;

FIG. 20 is a flowchart depicting example operation of voltagemaintenance for a node;

FIG. 21 is a flowchart depicting example operation of an alarm circuit;

FIG. 22A is a flowchart depicting example operation of capacitorcharging;

FIG. 22B is a flowchart depicting example operation of pump strokemonitoring;

FIG. 23 is a flowchart depicting example operation of a low batterycontrol process; and

FIG. 24 is a flowchart depicting example control of telemetry circuitry.

DETAILED DESCRIPTION

An infusion device according to the principles of the present disclosuremay use a solenoid-operated piston pump for pumping a drug to a patientinstead of a miniature stepper motor as used in previous systems. Whilethe miniature stepper motor had a peak current demand of, for example, 5milliamps, a piston pump may have a higher peak current demand, such as0.5 amps. The difference between those peak current demands is twoorders of magnitude, creating a much more difficult current draw for abattery to source.

In addition, battery chemistry may cause the battery to have a largeseries resistance, and the infusion device may include many loads on thebattery, such as pump cap charge, alarm, telemetry and electronic portfinding. Further, there may be minimal voltage headroom between therequired operating voltage of various components and the voltageprovided by the battery, especially as the battery output decreases overtime.

As a result, the present disclosure describes a variety of approaches,including exclusivity, interleaving, and duty-cycling, to maximizebattery life, minimize voltage droop, and allow a range of demands to bemet by a single battery. For example, regulators may require a certaininput level to allow the output to be regulated. When the supply voltagedips below that level, the output voltage is no longer regulated and maysimply track the battery voltage. In addition, telemetry and digitallogic may require a certain voltage level to operate deterministically.The following description presents solutions to satisfy theserequirements while still providing a large current to actuate a pistonsolenoid.

Referring now to FIG. 2, an example implantable infusion device 200includes a battery 204, whose output is connected to a protectioncircuit 208. The protection circuit may limit the amount of currentsupplied by the battery 204 to prevent undesirable heat increases fromexcessively high current draw such as may be caused by a short circuit.An output of the protection circuit 208 is referred to as BPLUS, whichis functionally the battery voltage for the remainder of the implantableinfusion device 200.

A pump 212 delivers amounts of drug from a reservoir 216 to a catheter220. The implantable infusion device 200 may be implanted in anabdominal cavity of a patient and the catheter 220 provides the drug toan intrathecal space or an epidural space of the patient. The pump 212includes a solenoid coil 224, which is energized by a pump actuatorcircuit 228. Energizing the solenoid coil 224 moves a piston 226, whichmechanically forces an amount of the drug into the catheter 220. Aportion of the piston 226 may be an armature surrounded by the solenoidcoil 224.

The pump actuator circuit 228 may energize the solenoid coil 224 from avoltage rail labeled CAP. The CAP voltage rail is supported by chargestorage 232. The charge storage 232 may include one or more capacitorsin parallel, such as a bank of four capacitors totaling four thousandmicrofarads.

The pump actuator circuit 228 may energize the solenoid coil 224 oncethe voltage of the CAP voltage rail has reached a predetermined voltage.For example, this predetermined voltage may be 2.8 V. A charging circuit236 supplies current to the charge storage 232. Although the chargestorage 232 is not limited to capacitors, for ease of explanation,delivering charge to the charge storage 232 will be referred to in thisapplication as charging the capacitor 232.

At various times, the voltage to which the CAP voltage rail is desiredto be charged is close to or even greater than the battery voltageBPLUS. In those situations, the voltage boost circuit 240 provides anincreased voltage, labeled in FIG. 2 as BATT2X. BATT2X and BPLUS arereceived by a multiplexer circuit 244. Digital circuitry 248 selectseither BPLUS or BATT2X to be provided to the charging circuit 236. Byboosting BPLUS, the charging circuit 236 has enough voltage headroom tocharge the capacitor 232 to the desired voltage.

In various implementations, the voltage boost circuit 240 may include avoltage doubler and/or a voltage tripler. Although the output of thevoltage boost circuit 240 is labeled BATT2X, the voltage boost circuit240 is not limited to only doubling the voltage BPLUS. The voltage boostcircuit 240 may boost the voltage BPLUS by other factors, and mayprovide more than one boost level. The voltage boost circuit 240 may bereferred to as a DC-DC converter, and may be implemented using one ormore charge pumps.

An alarm circuit 252 is also powered from the voltage boost circuit. Invarious implementations, the alarm circuit 252 may be powered by avoltage, BATT3X, that is higher than BATT2X. For example, BATT3X may beapproximately equal to three times BPLUS. The voltage boost circuit 240may produce BATT2X and BATT3X mutually exclusively. In variousimplementations, the alarm circuit 252 may drive the alarm (not shown)with out-of-phase signals, both having voltage swings between zero andapproximately BATT3X. If the out-of-phase signals are connected to thealarm with reverse polarity, the total voltage swing seen by the alarmmay be approximately two times BATT3X. In this manner, the peak-to-peakvoltage across the alarm may be, for example, approximately six timesBPLUS.

The alarm circuit 252 generates an alarm that can be used to signal anevent, such as an approaching end of life of the battery 204, or aproblem with the pump 212. In various implementations the alarm circuit252 can provide feedback to indicate that transmitted information, suchas a new dosing schedule, has been received by the implantable infusiondevice 200. The alarm circuit 252 can also signal events, such as thebeginning of a bolus.

The alarm circuit 252 is made to be perceptible to the patient and mayalso be made to be perceptible to third parties, which may be relevantin a clinical setting or if the patient is incapacitated. For example,the alarm circuit 252 may include an audible alarm and/or a vibratoryalarm.

A charge monitor circuit 256 monitors the voltage on the CAP voltagerail, which the digital circuitry 248 uses to determine when there issufficient voltage for the pump actuator circuit 228 to energize thesolenoid coil 224. The charge monitor circuit 256 may also determinewhen the voltage of the CAP voltage rail is close enough to BPLUS thatvoltage boosting is necessary to effectively continue charging thecapacitor 232.

A battery monitor circuit 260 monitors BPLUS and determines whetherBPLUS falls below various thresholds during various operatingconditions. These determinations may be acted upon by digital circuitry248. For example, while charging the capacitor 232, the battery monitorcircuit 260 may generate a trip signal when BPLUS decreases below afirst threshold. This trip signal may be used to activate the voltageboost circuit 240. The first threshold may be derived from a referencevoltage generated by a reference such as a bandgap reference circuit262.

When either charging is occurring or a telemetry circuit 264 isoperating, the battery monitor circuit 260 may compare BPLUS to a secondthreshold. If BPLUS decreases below that threshold, charging of thecharge storage 232 may be slowed, paused, or halted. Further, whentelemetry is operating, the battery monitor circuit 260 may compareBPLUS to a third threshold and if BPLUS decreases below that thirdthreshold, the battery monitor circuit 260 may signal that telemetryshould be shut down until BPLUS recovers. The telemetry circuit 264allows communication to and/or from the implantable infusion device 200,such as with an external programmer or data logger.

To provide a relatively stable voltage supply to various components,such as analog regulators 268 and digital regulators 272, a voltage raillabeled BPHOLD is used. The digital regulators 272 may provide aregulated voltage to the digital circuit 248. BPHOLD is selectivelyconnected to BPLUS by isolation circuit 276. The isolation circuit 276disconnects BPHOLD from BPLUS in response to a comparison circuit 280.The comparison circuit 280 may instruct the isolation circuit 276 todisconnect BPHOLD from BPLUS when BPLUS is not at least a predeterminedvoltage greater than BPHOLD. For example only, the comparison circuit280 may isolate BPHOLD from BPLUS when BPLUS is not at least twentymillivolts greater than BPHOLD. Digital circuitry 248 may be configuredto selectively disable the comparison circuit 280, thereby causing theisolation circuit 276 to disconnect BPHOLD from BPLUS regardless of whatis measured by the comparison circuit 280.

The voltage of BPHOLD is maintained by charge storage 284. The chargestorage 284 may include one or more capacitors, which may total, forexample, one hundred microfarads. A hold monitor circuit 288 monitorsthe voltage of BPHOLD and generates a BPHOLD_High signal when thevoltage of BPHOLD exceeds a threshold. Similarly, the hold monitorcircuit 288 may generate a BPHOLD_Low signal when the voltage of BPHOLDdecreases below a second threshold. In response to the BPHOLD_Low signalbeing generated, the digital circuitry 248 may activate the voltageboost circuit 240, which supplies boosted voltage to BPHOLD via arecharge circuit 292.

The current from the voltage boost circuit 240 will charge the chargestorage 284 until the hold monitor circuit 288 generates the BPHOLD_Highsignal, at which point the recharge circuit 292 turns off. The digitalcircuitry 248 may then also turn off the voltage boost circuit 240. Thecharge storage 284 then supplies current to loads on the BPHOLD rail,causing the voltage to decrease to BPHOLD_Low, at which point therecharge cycle starts again. A CPU monitor circuit 296 monitors aprocessor voltage and may inform the digital circuitry 248 of a resultof a comparison of the processor voltage with a threshold.

Alternatively to providing current to the charge storage 232, thecharging circuit 236 may power an electronic port finder circuit 300.The electronic port finder circuit 300 may provide electromagneticsignals that allow a refill operation to accurately locate a refill port304 on the reservoir 216. The charging circuit 236 may supply current toeach of three locating coils of the electronic port finder circuit 300.The electromagnetic field created by the locating coils is used by alocation apparatus during a refill procedure.

Referring now to FIG. 3, an example implementation of the protectioncircuit 208 may simply include a resistor 320. The resistance of theresistor 320 may be chosen to satisfy a temperature rise constraint inthe event of a short circuit. In other words, the resistance value ofthe resistor 320 may be set to limit the amount of current flowing in ashort circuit to a level that will cause the temperature of theimplantable infusion device 200 to rise at a lower rate than thespecified temperature rise rate. For example only, the resistance valuemay be 10 ohms. In various other implementations, the protection circuit208 may be implemented using an active circuit that controls and limitscurrent in the event of a short circuit. When no short circuit ispresent, the active circuit may present a very low resistance to currentflow from the battery 204.

Referring now to FIG. 4, an equivalent schematic of the battery 204 isshown. The electrochemical cells produce a DC voltage 340, which forexample may start out at approximately 3.1 V and decrease over time toapproximately 2.6 V or less. Intrinsic resistance of the battery may bemodeled as series resistor 344. For example only, the resistor 344 mayhave a resistance value between thirty-five ohms and seventy-five ohms.A capacitor 348 may be added to provide capacity for current inrushevents. For example, the capacitor 348 may have a capacitance oftwenty-two microfarads.

Referring now to FIG. 5, an example implementation of the multiplexercircuit 244 is shown. BATT2X is received at a first terminal of atransistor 360, while BPLUS is received at a first terminal oftransistor 364. Second terminals of the transistors 360 and 364 areconnected to each other and are output from the multiplexer circuit 244.A control terminal of the transistor 364 is connected to a select input,which may be received from the digital circuitry 248. In variousimplementations, the select input may be an active-high BATT2X selectsignal—i.e., when the select signal is high, BATT2X is selected by themultiplexer circuit 244. An inverter 368 inverts the select signal andprovides the inverted select signal to a control terminal of thetransistor 360. Level shifters may be used, for example, to scale theoutput of the inverter 368 to the BATT2X voltage level. This preventsinsufficient voltages from causing unwanted operation: for example, ifthe control terminal of the transistor 360 falls below BATT2X, thetransistor 360 will begin to turn on.

In various implementations, the transistors 360 and 364 may be p-channelmetal oxide semiconductor field effect transistors (MOSFETs). Wellswitches (not shown) may be connected across each of the transistors 360and 360 to ensure that the body voltages are always connected to thehighest terminal potential in order to keep the well diodes frombecoming forward biased and turning on.

Referring now to FIG. 6, the isolation circuit 276 may be implementedusing a transistor 380 whose control terminal is controlled by thecomparison circuit 280 of FIG. 2. First and second terminals of thetransistor 380 are connected to BPLUS and BPHOLD, respectively. In theimplementation shown, the transistor 380 is a p-channel MOSFET and abody of the transistor 380 is connected to BPHOLD.

Referring now to FIG. 7, an example implementation of the rechargecircuit 292 includes a transistor 400 whose control terminal iscontrolled by the digital circuitry 248. First and second terminals ofthe transistor 400 are connected to BPHOLD and BATT2X, respectively. Awell switch 404 is connected to BPHOLD, BATT2X, and a body of thetransistor 400.

Referring now to FIG. 8, an example implementation of the batterymonitor circuit 260 is shown. A threshold circuit 420 compares BPLUS toa reference voltage and outputs a Doubler_Trip signal in response to thecomparison. To save power, the threshold circuit 420 may only operate atcertain times, such as when a charging mode is activated as indicated bythe Charge_Mode signal. The charging mode is enabled when the chargingcircuit 236 is charging the capacitor 232 in preparation for actuatingthe pump 212.

The Doubler_Trip signal may be operated on by a filter 424. For exampleonly, the filter 424 may determine when the Doubler_Trip signal isasserted for at least a predetermined period of time, at which point thefilter 424 outputs a Doubler_Fault signal. In other words, the filter424 removes glitches of the Doubler_Trip signal that are shorter thanthe predetermined period of time. In various implementations, thepredetermined period of time may be twenty milliseconds. The thresholdcircuit 420 may allow an adjustable threshold to be set, such as isshown in FIGS. 9 and 10. For example only, the threshold circuit 420 mayallow VBPLUS to be compared to a voltage within the range of 2.2 V to2.45 V. The Doubler_Fault signal may cause the voltage boost circuit 240of FIG. 2 to be activated.

A threshold circuit 428 compares VBPLUS to VREF when either telemetry ison, as indicated by the Telemetry_On signal, or the charge mode isenabled. In response to this comparison, a Low_Battery signal isgenerated. For example only, the Low_Battery signal may be generatedwhen VBPLUS is less than a predetermined voltage, which for example canbe set within the range of 1.9 V to 2.1 volts. The Low_Battery signalmay cause charging of the capacitor 232 to be slowed, paused, or halted.

A threshold circuit 432 compares VBPLUS to a reference voltage when theTelemetry_On signal is present and in response, generates aTelemetry_Power signal. For example, the threshold circuit 432 generatesthe Telemetry_Power signal when VBPLUS is less than the voltage in arange of 1.8 V to 1.9 V. The Telemetry_Power signal may be used to turnoff and/or reset the telemetry circuit 264 of FIG. 2.

Referring now to FIG. 9, an example implementation of a thresholdcircuit 460 is shown. The threshold circuit 460 may be implemented inone or more of the threshold circuits 420, 428, and 432 of FIG. 8. Thethreshold circuit 460 includes an operational amplifier (opamp) 464 thatoperates only when an enable signal EN is present. The opamp 464 has anon-inverting input that receives a voltage from a node of a firstvoltage divider connected to a voltage to be measured, V_(MEASURE). Aninverting input of the opamp 464 is connected to a node of a secondvoltage divider, which is connected to a reference voltage, such as fromthe bandgap reference circuit 262.

The first voltage divider includes a resistor 468 and a resistor 472.Although FIG. 9 shows that the resistor 468 is a variable resistor,either, both, or none of the resistors 468 and 472 may be variable. Whenthe same voltage is going to be measured by multiple threshold circuitsincluding the threshold circuit 460, the first voltage divider may beshared between the threshold circuits. The second voltage dividerincludes a resistor 476 and a resistor 480. In various implementations,either, both, or none of the resistors 476 and 480 may be variableresistors.

Referring now to FIG. 10, one example implementation of a variableresistor 500 is shown. For example only, the variable resistor 500 maybe used to implement any of the resistors 468, 472, 476, and 480 in FIG.9. The variable resistor 500 includes fixed resistors 504, 508, 512, and516 in series. Transistors 520, 524, and 528 selectively short circuitthe resistors 504, 508, and 512, respectively.

The transistors 520, 524, and 528 are controlled by a control register532. The value stored by the control register 532 can therefore set thetotal resistance of the variable resistor 500. Although three switchableresistors are shown in FIG. 10, more or fewer may be used in any givenimplementation of a variable resistor. The resistor 516, which is notswitchable, is included in FIG. 10 to offset the possible values ofresistance. Therefore, even if the control register 532 causes each ofthe resistors 504, 508, and 512 to be short circuited, the variableresistor 500 will still have the resistance of the resistor 516. Toafford the maximum range of control, the resistors 504, 508, and 512,may have a binary relationship: for example, the resistor 508 has aresistance value twice that of the resistor 504 and the resistor 512 hasa resistance value twice that of the resistor 508.

Referring now to FIG. 11, an example implementation of the chargemonitor circuit 256 is shown. A threshold circuit 560 compares thevoltage, VCAP, of the CAP voltage rail to a reference voltage when theCharge_Mode signal is present. The threshold circuit 560 generates aMin_Charge signal when VCAP is greater than the reference voltage, suchas a voltage in the range of 2.7 V to 2.9 V.

A threshold circuit 564 compares VCAP to VBPLUS when the Charge_Modesignal is present. For example, the threshold circuit 564 generates aMax_Charge signal when VCAP is greater than a predetermined percentageof VBPLUS. The predetermined percentage may be variable within a range,such as between 92 percent and 99 percent. A filter 568 may output aMax_Charge_Filter signal in response to the Max_Charge signal beingpresent for at least a predetermined period of time. For example only,the predetermined period of time may be twenty milliseconds.

The Max_Charge_Filter signal indicates that VCAP is getting too close toVBPLUS and therefore the voltage boost circuit 240 should be enabled tocontinue charging the capacitor 232. The Min_Charge signal indicatesthat the minimum level of charging necessary for actuating the pump 212has been achieved and therefore the pump can be actuated.

Referring now to FIG. 12, an example implementation of the CPU monitorcircuit 296 is shown. A threshold circuit 580 compares a CPU voltage toa reference voltage based on an enable signal. For example, the enablesignal may be controlled by a firmware register. An inverter 584 invertsan output of the threshold circuit 580 and outputs a VCPU_OK signal.

Referring now to FIG. 13A, an example implementation of the hold monitorcircuit 288 is shown. A threshold circuit 604 compares the voltage ofBPHOLD, VBPHOLD, to a reference voltage while an enable signal ispresent. The output of the threshold circuit 604 serves as the enablesignal for a threshold circuit 608 and is input to an inverter 612. Anoutput of the inverter 612 is a BPHOLD_High signal indicating thatrecharging of the BPHOLD from the voltage boost circuit 240 can stop.For example only, the threshold circuit 604 may compare VBPHOLD to arange between 2.4 V and 2.8 V.

The threshold circuit 608 compares VBPHOLD to a threshold voltage andoutputs a BPHOLD_Low signal in response. The BPHOLD_Low signal indicateswhen the voltage of BPHOLD has dropped too low and recharging isnecessary. For example only, the threshold circuit 608 compares VBPHOLDto a voltage in the range of 2.2 V to 2.4 V.

Referring now to FIG. 13B, another example implementation of the holdmonitor circuit 288 is shown. A threshold circuit 620 generates aBPHOLD_High signal when VBPHOLD is greater than a reference voltage. Athreshold circuit 624 generates a BPHOLD_Low signal when VBPHOLDdecreases below another reference voltage. A state machine circuit 628controls the voltage boost circuit 240 and/or the recharge circuit 292in response to the BPHOLD_High signal and the BPHOLD_Low signal, asdescribed in more detail below.

In a significantly simplified system, the state machine circuit 628could be implemented as an SR latch, which receives the BPHOLD_Lowsignal at a set input and receives the BPHOLD_High at a reset input. Anoutput Q of the latch would be provided to the recharge circuit 292 andthe voltage boost circuit 240. Therefore, when the BPHOLD_Low signal isgenerated, the latch 628 outputs an enable signal to enable the voltageboost circuit 240 and the recharge circuit 292. This charges up BPHOLDusing the boosted voltage. Once the BPHOLD_High signal is generated,resetting the latch 628, the output Q of the latch 628 disables therecharge circuit 292 and the voltage boost circuit 240, ending therecharging.

Referring now to FIG. 14, an example implementation of the chargingcircuit 236 is shown. A reference block 650 generates a referencecurrent I_(REF) 654 through a transistor 658. Additionally, oralternatively, the transistor 658 may be cascoded with one or moreadditional transistors to improve power supply rejection. In variousimplementations, I_(REF) 654 may be generated using a resistor.Alternatively, a temperature-compensated or temperature-independentcurrent generator may be used. However, the temperature within thepatient may vary within a small range, making a resistor an acceptablecurrent generator.

The reference current through the transistor 658 is mirrored in a mirrorblock 660 through N current mirror transistors 662-1, 662-2, . . . 662-N(collectively, current mirror transistors 662). The control terminals ofeach of the current mirror transistors 662 may be selectively connectedto the control terminal of the transistor 658 via pass transistors670-1, 670-2, . . . 670-N (collectively, pass transistors 670). The passtransistors 670 are respectively controlled by control lines Control₁,Control₂, . . . Control_(N) (collectively, control lines). Theaggregated current from the current mirror transistors 662 is referredto as I_(OUT), and the values of the control lines therefore control howlarge I_(OUT) is. The mirror block 660 provides I_(OUT) to the capacitor232.

In various implementations, the current mirror transistors 662 may besized in a binary fashion to allow for a high dynamic range of control.The I_(OUT) generated by the mirror block 660 may vary, for example,from 0.5 milliamps to 5.75 milliamps. The number of transistors currentmirror transistors 662 may be one or may be any integer greater thanone. When there is only one current mirror transistor 662, the level ofI_(OUT) may be regulated using a different mechanism, such as byadjusting I_(REF) 654.

For operating the electronic port finder circuit 300, an electronic portfinder mirror 680 may be included. The electronic port finder mirror 680may include a transistor 682 whose control terminal is connected to thecontrol terminal of the transistor 658. In practical implementations,additional current mirror legs may be present, and the current referencemay be transferred to the transistor 682 via additional mirrortransistors, such as NMOS mirror transistors.

In various implementations, the charging circuit 236 may include aplurality of transistors 690 to selectively provide current to differenttransmit coils of the electronic port finder circuit 300 from theelectronic port finder mirror 680. For example, transistors 690-1,690-2, and 690-3 may be controlled in a mutually exclusive fashion toprovide current to respective coils of the electronic port findercircuit 300.

In various implementations, the electronic port finder circuit 300 maybe operated at only BPLUS, not at VMUX from the multiplexer circuit 244,which will be either BPLUS or BATT2X. Because BPLUS is a lower voltage,the components of the electronic port finder mirror 680 may be connecteddirectly to BPLUS instead of VMUX. The components, such as thetransistor 682, may be implemented as devices having lower maximumvoltage limits, which may allow the components to be smaller in size orexhibit other advantages. In various implementations, charging thecapacitor 232 may be mutually exclusive to operating the electronic portfinder circuit 300.

Referring now to FIG. 15, an example functional block diagram of thedigital circuitry 248 is presented. The digital circuitry 248 is shownin FIG. 15 including analog-to-digital converters 700 even though suchconverters are not strictly digital. The analog-to-digital converters700 include a converter 700-1 that receives the Doubler_Trip signal, aconverter 700-2 that receives the Doubler_Fault signal, a converter700-3 that receives the Low_Battery signal, a converter 700-4 thatreceives the Telemetry_Power signal, a converter 700-5 that receives theMin_Charge signal, a converter 700-6 that receives the Max_Chargesignal, a converter 700-7 that receives the BPHOLD_High signal, and aconverter 700-8 that receives the BPHOLD_Low signal.

The digital values are received by a processor 704, which operates usingnonvolatile memory 708, which may include firmware, and volatile memory712, which may include temporary storage. For example only, thenonvolatile memory 708 may include flash memory and/or read-only memory,while the volatile memory 712 may include random access memory. Theprocessor 704 generates various outputs to control other elements of theimplantable infusion device 200. In various implementations, theprocessor 704 may be supplemented by custom or programmable logic thatperforms functions otherwise described as being performed by theprocessor 704. Various inputs and various outputs of the processor 704may therefore be received/outputted by the custom or programmable logic.

Referring now to FIG. 16, a simplified functional block diagram of anexample implementation of the processor 704 is shown. Although theprocessor 704 is shown in FIG. 16 to include the following circuits,these circuits may be implemented as discrete circuitry either inside oroutside of a processor, such as in a system-on-chip or applicationspecific integrated circuit. Further, in accordance with the definitionof circuits set out below, these circuits are not necessarily eachseparate, mutually exclusive, hardware elements. In variousimplementations, code executed by the processor 704 may be replaced bydigital state machines implemented in custom logic or programmablelogic.

A pump control circuit 750 determines when the pump 212 should beactuated and instructs a charging control circuit 754 to store up enoughcharge to actuate the pump 212. In various implementations, the pumpcontrol circuit 750 receives the Min_Charge signal when enough charge ispresent, as evidenced by a capacitor voltage reaching a predeterminedthreshold. Once enough charge is present and a pump stroke is desired,the pump control circuit 750 outputs a signal to the pump actuatorcircuit 228 to energize the solenoid coil 224. The pump control circuit750 may additionally or alternatively operate in response to theMax_Charge signal.

The charging control circuit 754 also receives the Min_Charge signal,which indicates that charging of the capacitor 232 can be stopped. Thecharging control circuit 754 also receives the Low_Battery signal, whichindicates that charging should be slowed, paused, or stopped, asdescribed in more detail below. The charging control circuit 754 alsoreceives the Max_Charge signal, which indicates that a boosted voltageis necessary to effectively continue charging the charge storage 232.The charging control circuit 754 then instructs a boost control circuit758 to turn on the voltage boost circuit 240 to provide the additionalvoltage coverage needed. The charging control circuit 754 also providesmultiplexer control to the multiplexer circuit 244 to select the boostedvoltage from the voltage boost circuit 240.

The charging control circuit 754 controls the amount of charging currentgenerated by the charging circuit 236. This charging current may bepartially reduced or decreased to zero in response to the Low_Batterysignal.

An electronic port finder control circuit 762 controls the chargingcircuit 236 to output current to the electronic port finder circuit 300and includes three signals EPF2, EPF1, and EPF0 to select which of thecoils will be energized. The electronic port finder control circuit 762also disables the charging control circuit 754 and the boost controlcircuit 758 while electronic port finding is being performed.

A voltage maintenance circuit 766 receives the BPHOLD_High andBPHOLD_Low signals. In response to these signals, the voltagemaintenance circuit selectively enables the boost control circuit 758and provides a recharge control signal to the recharge circuit 292 ofFIG. 2. While the BPHOLD bus is recharging, the voltage maintenancecircuit 766 may inhibit the charging control circuit 754.

An alarm control circuit 770 determines when an alarm should be soundedand, in response, instructs the boost control circuit 758 to enable thevoltage boost circuit 240 of FIG. 2. At the same time, the alarm controlcircuit 770 inhibits the charging control circuit 754. Once startup ofthe voltage boost circuit 240 is complete, the alarm control circuit 770instructs the alarm circuit 252 to generate the alarm sound.

A telemetry control circuit 774 receives the Telemetry_Power signal and,in response to the Telemetry_Power signal, turns off telemetry. Once theTelemetry_Power signal is no longer present, the telemetry controlcircuit 774 recognizes that the battery voltage BPLUS has recovered andtelemetry can be turned back on. The telemetry control circuit 774 canalso temporarily disable telemetry, such as based on an inhibit requestfrom the boost control circuit 758.

The boost control circuit 758 controls the voltage boost circuit 240 ofFIG. 2 based on a boost request from the alarm control circuit 770, thevoltage maintenance circuit 776, or the charging control circuit 754.The boost control circuit 758 also can initiate boost based on theDoubler_Trip signal and/or the Doubler_Fault signal.

When the voltage boost circuit 240 is started, a larger in-rush currentmay occur before a lower steady-state current is established. Duringthis in-rush time, the boost control circuit 758 may inhibit the alarmcontrol circuit 770 from activating the alarm, may inhibit telemetry viathe telemetry control circuit 774, may inhibit electronic port findingvia the electronic port finder control circuit 762, and may inhibitcharging via the charging control circuit 754.

An isolation control circuit 778 selectively outputs a force isolationsignal to the comparison circuit 280 of FIG. 2, which causes theisolation circuit 276 of FIG. 2 to isolate BPHOLD from BPLUS. This maybe done for testing purposes.

Referring now to FIG. 17, a simplified functional block diagram of anexample implementation of the pump control circuit 750 is presented. Adose determination circuit 804 determines how much drug per unit of timewill be delivered to the patient. This may be based on values providedby a clinician and/or by the patient themself. This value may remainrelatively constant throughout the day, but may be interrupted by one ormore boluses which may be higher doses of drug provided at certain timesof the day. In addition, boluses may be delivered at certain timesduring a week or a month and/or may be delivered while the patient is ina clinical setting.

The dose determination circuit 804 may have a number of specialoperating modes, such as a post-manufacturing mode, in which sterilesolution stored in the reservoir 216 during manufacturing is very slowlypumped out to keep all of the parts of the implantable infusion device200 operating correctly while the implantable infusion device 200 isshipped, stored, and sold. In addition, the dose determination circuit804 may have a mode in which a very high dosage rate is produced toprime the catheter, such as during the operation to implant theimplantable infusion device 200 in a patient.

Based on the determined dose from the dose determination circuit 804, apump timing circuit 808 determines an average time between pistonstrokes of the pump in order to achieve the desired dosage. The timingregulation circuit 812 controls the pump actuator circuit 228 of FIG. 2and the charging control circuit 754 of FIG. 16 to achieve the averagetiming specified by the pump timing circuit 808. For various reasons,such as a low battery voltage or other higher-priority current demands,charging the capacitor 232 may take longer than the desired averagetime. The timing regulation circuit 812 takes this into account andtherefore reduces the amount of time for the next piston stroke toattempt to deliver the appropriate amount of drug per unit of time.

The timing regulation circuit 812 provides a timer length value to atimer circuit 816, specifying the desired time until the next pumpstroke. The timer circuit 816 counts down (or up, depending on theimplementation), and before the timer is going to expire, the timercircuit 816 instructs the charging control circuit 754 to begin chargingthe capacitor 232. The timer circuit 816 may learn over time how longthis charging process takes and actuate the charging control circuit 754at the appropriate time before expiration of the timer. The timercircuit 816 may take into account other current loads, such astelemetry, when determining how soon charging needs to begin.

When the timer expires, the timer circuit 816 provides an expire signalto an AND gate 820. The AND gate 820 outputs an energize piston signalto the pump actuator circuit 228 when both the expire signal is receivedand the Min_Charge signal is present. For example only, the pumpactuator circuit 228 may include one or more transistors arranged inparallel that rapidly discharge the charge storage 232 into the solenoidcoil 224 of the pump 212 in order to actuate the piston and pump thedrug through the catheter 220 and into the patient.

Referring now to FIG. 18, a plot of various control signals over time ispresented. Traces include an electronic port finder enable signal 904,an electronic port finder enable register 908, a first timer 912, adelayed first timer 916, a second timer 920, a Doubler_Startup signal924, a time to charge signal 928 for the BPHOLD rail, an XTELM_HIDDsignal 932, and a PWR_INH signal 936.

The signals 904 and 908 are active high to enable electronic port finderoperation. The signal 912 is active high to delineate when electronicport finding is operated. While the signal 912 is high, interleaving ofthe electronic port finder coils is shown, with a blank intervalfollowed by coil 0, followed by coil 1, and followed by coil 2. Thispattern repeats while timer_1 remains asserted. Alternatively, thepattern could have the electronic port finder coils energized in adifferent order, and the blank period could be at a different location,such as after each time the three electronic port finder coils wereenergized instead of before. Additional information on interleavingcoils, the gaps between the coils, and the blank periods can be found incommonly assigned application Ser. No. 13/045,683, filed Mar. 11, 2011,the disclosure of which is hereby incorporated by reference in itsentirety.

The delayed timer signal 916 is not de-asserted until a predeterminedperiod of time after the signal 912 is de-asserted. This is used toinhibit telemetry so that there is no interference between electronicport finding and telemetry. The predetermined period may, for example,be 50 microseconds.

The signal 920 is inverted compared to the signal 912 and is active highto indicate that other functions can be performed now that electronicport finding operation has paused. The signal 924 indicates that thevoltage boost circuit 240 of FIG. 2 is starting up, a period duringwhich various functionality, including telemetry, is inhibited. In thisexample, startup is shown to take 12 milliseconds.

The reason for the doubler starting up is shown in signal 928, whichdepicts the amount of time necessary to charge the BPHOLD bus from theboosted voltage created by the voltage boost circuit 240. This indicatesthat the voltage of BPHOLD had decreased too far and needed to beboosted. For example only, the charging time may be 33 milliseconds.This may be a fixed number or may be a measurement of the actualcharging time. The signal 932 enables telemetry and may disabletelemetry whenever signal 912 is high. The signal 936 exerts anadditional inhibit on telemetry, which lasts until after BPHOLD ischarged up (at the end of the 33 millisecond window of signal 928).

Referring now to FIG. 19, a flowchart depicts example operation of theboost control circuit 758 of FIG. 16. Control begin at 1004, wherecontrol determines whether a Doubler_Trip signal or a Doubler_Retrysignal is present. If so, control continues at 1008; otherwise, controlreturns to 1004. At 1008, control enables a clock for use by the voltageboost circuit 240 of FIG. 2. For example only, the clock may run at 100KHz.

Control continues at 1012, where control determines whether theDoubler_Retry signal is present. If so, control transfers to 1016;otherwise, control continues at 1020, where control resets and starts atimer. The timer is for a predetermined period of time that eliminatesglitches in the Doubler_Trip signal, such as twenty milliseconds.Control continues at 1024, where control determines whether theDoubler_Trip signal is still present. If so, control transfer to 1028;otherwise, control returns to 1004.

At 1028, control determines whether the timer has expired. If so,control continues at 1032; otherwise, control returns to 1024. At 1032,control signals a Doubler_Fault and continues at 1016. At 1016, controlinhibits telemetry and continues at 1036, where capacitor charging isalso stopped. Control continues at 1040, where the voltage boost circuitis started and continues at 1044, where control waits for startup of thevoltage boost circuit to complete. Control continues at 1048, wheretelemetry is re-enabled, and continues at 1052, where BATT2X is selectedby the multiplexer circuit 244 of FIG. 2 for charging.

Control continues at 1056, where capacitor charging is resumed.Throughout FIG. 19, while the capacitor is being charged, a Low_Batteryevent may delay or stop the capacitor charging. For example only, acapacitor charging back-off process, such as is described in FIG. 23below, may be implemented.

Control continues at 1060, where control determines whether theMin_Charge signal is present. If so, control continues at 1064;otherwise, control remains in 1060. At 1064, control disables the clock,and at 1068, control selects BPLUS for the charging circuit 236 usingthe multiplexer circuit 244. Control continues at 1072, where thevoltage boost circuit is disabled, and control continues at 1076.Control remains at 1076 until the pump stroke is completed, and thencontrol returns to 1004. In various implementations, the pump stroke maysometimes be significantly delayed after the Min_Charge signal ispresent. If so, the voltage on the capacitor may decrease over thatdelay, and therefore control may return to 1016 to restart the voltageboost circuit and recharge the capacitor up to the required level.

Referring now to FIG. 20, a flowchart depicts example operation of thevoltage maintenance circuit 766 of FIG. 16. Control begins at 1104,where control determines whether the BPHOLD_Low signal is present. Ifso, control transfers to 1108; otherwise, control returns to 1104. At1108, control enables a clock for the voltage boost circuit. Controlcontinues at 1112, where telemetry is inhibited. For example only,telemetry may be inhibited by making a telemetry inhibit request, whichis granted once an ongoing telemetry operation completes.

Control continues at 1116, where the voltage boost circuit is started.Control continues at 1120 where control waits for startup of the voltageboost circuit to complete. Control continues at 1124, where telemetry isenabled, and continues at 1128, where capacitor charging is inhibited.Control continues at 1132, where BPHOLD is charged from BATT2X. Controlcontinues at 1136, where control determines whether the BPHOLD_Highsignal is present. If so, control continues at 1140; otherwise, controlremains at 1136. At 1140, control stops charging BPHOLD from BATT2X andcontrol continues at 1144. At 1144, control disables the voltage boostcircuit. Control continues at 1148, where capacitor charging isre-enabled and control continues to 1104.

Referring now to FIG. 21, example operation of the alarm control circuit770 of FIG. 16 is presented. Control begins at 1204, where controldetermines whether an alarm request is present. If so, control continuesat 1208; otherwise, control remains at 1204. At 1208, control enables aclock for operating the voltage boost circuit 240 of FIG. 2. Controlcontinues at 1212, where telemetry is inhibited, and continues at 1216,where the voltage boost circuit is started. Control continues at 1220,where control waits for startup of the voltage boost circuit to becompleted. Control continues at 1224, where telemetry is enabled, andcontinues at 1228, where capacitor charging is inhibited. Controlcontinues at 1232, where the alarm is sounded. Control continues at1236, where control determines whether the request for an alarm is over.If so, control transfers to 1240; otherwise, control remains at 1236. At1240, control disables the alarm and continues at 1244, where thevoltage boost circuit is disabled. Control continues at 1248, wherecapacitor charging is enabled, and returns to 1204.

Referring now to FIG. 22, a flowchart depicts example charging controlperformed by the pump control circuit 750 and the charging controlcircuit 754 of FIG. 16. Control begins at 1304, where a desired doserate is determined. Control continues at 1308, where a desired pumpstroke interval is determined in response to the dose rate of 1304 aswell as recorded information concerning the doses delivered so far.

Control continues at 1312, where a pump timer is set in response to theinterval of 1308. Control continues at 1316, where control determineswhether the pump timer is close to expiring. If so, control transfers to1320; otherwise, control remains at 1316. At 1320, control beginscharging the capacitor 232. Control continues at 1324, where if theMax_Charge signal is present, control transfers to 1328; otherwise,control transfers to 1332.

At 1328, control enables the clock for the voltage boost circuit 240 ofFIG. 2. Control continues at 1336, where control sets and starts afilter timer. Control continues at 1340, where if the Max_Charge signalis still present, control continues at 1344; otherwise, controltransfers to 1332. At 1344, control determines whether the filter timerhas expired. If so, control transfers to 1348; otherwise, controlreturns to 1340. At 1348, control signals Max_Charge_Filter andcontinues at 1352, where telemetry is inhibited. Control continues at1356, where the voltage boost circuit is started. Control continues at1360, where control waits for the startup to finish.

Control then continues at 1364, where telemetry is enabled, andcontinues at 1368, where BATT2X is selected for charging the chargestorage 232. Control then resumes charging the capacitor 232 at 1372.Control continues at 1376, where if the Max_Charge signal is stillpresent, control transfers to 1380; otherwise, control continues at1384. At 1380, control determines whether the Min_Charge signal ispresent and, if so, control transfers to 1384; otherwise, controlreturns to 1376. At 1384, control selects BPLUS for charging thecapacitor 232 and continues at 1388, where the voltage boost circuit isdisabled.

Control continues at 1332, where if the Min_Charge signal is present,control transfers to 1392; otherwise, control returns to 1324. At 1392,control stops charging the capacitor 232 and continues at 1396. At 1396,control determines whether the pump timer has expired. If so, controltransfers to 1400; otherwise, control remains at 1396. At 1400, controlsends an energized signal to the pump actuator circuit 228 of FIG. 2.Control continues at 1404, where the pump actuator circuit 228 energizesthe solenoid coil and causes a stroke of the pump. As the pump solenoidis energized, the inductance of the solenoid stores electrical energy.In various implementations, some of this energy is recovered after thepiston is actuated, with the recovered energy being stored back on thecapacitor. See commonly assigned U.S. Pat. No. 7,927,326 to Sarkinen etal., issued Apr. 19, 2011, the disclosure of which is herebyincorporated by reference in its entirety. Control continues at 1408,where control updates delivered dose information to reflect the drugdelivered at 1404. Control then returns to 1304.

Referring now to FIG. 22B, a pump stroke catch-up process is shown. Thisprocess can be used in conjunction with or alternatively to elements ofFIG. 22A, such as 1308, 1312, and 1316. Control begins at 1420 wherecontrol initializes a counter to 0 and initializes a fault timer. Forexample, the fault timer may be initialized to zero and may then beginincreasing. Control continues at 1422, where control determines a gaptime between pump strokes based on the dosage rate. For example, thedosage rate may indicate that one pump stroke should be delivered eachminute, and therefore the gap time is set to one minute.

Control continues at 1424, where a stroke timer is set to the gap time.The stroke timer then begins decrementing. Once the stroke timer gets tozero, the stroke timer is considered expired, and another stroke will berequested. Control continues at 1426, where control checks whether thecounter is greater than zero. If so, control transfer to 1428;otherwise, control transfers to 1430.

At 1428, control requests a pump stroke and control continues at 1430.In various implementations, control requests a single pump stroke at atime and once that pump stroke has been delivered, the next pump strokeis requested. When the counter is greater than one and multiple pumpstrokes are desired, the pump strokes may be requested consecutively.

At 1430, control determines whether the stroke timer has expired. If so,control transfers to 1432; otherwise, control transfers to 1434. At1432, control increments the counter and continues at 1436. At 1436,control sets the stroke timer back to the gap time and continues at1434. At 1434, control determines whether a pump stroke has beendelivered. If so, control transfers to 1438; otherwise, controltransfers to 1440.

At 1438, control decrements the counter and continues at 1442. At 1442,control resets the fault timer and continues at 1440. The fault timermay track the amount of time since the last pump stroke was delivered.If this time is too great, this may indicate a failure, either ofsoftware or hardware. At 1440, control determines whether the faulttimer is greater than a threshold. If so, control transfers to 1444where a fault is signaled and control stops; otherwise, controltransfers to 1446.

The threshold may be a fixed value set to be greater than the maximumamount of time between any two pump strokes. In various implementations,the threshold may be based on the gap time. For example, if the gap timeis one minute, the fault timer threshold may be set to three minutes;meanwhile, if the gap time between pump strokes is five minutes, thethreshold may be set to seven minutes. Although these examples presentan offset of two minutes from the gap time, the offset may instead be apercentage of the gap time. Alternatively, another method of determiningthe threshold, which may be partially or completely based on the gaptime, may be used. At 1446, control determines whether a change in thedosage rate has been requested. If so, control returns to 1422, where anew gap time is determined; otherwise, control returns to 1426.

As shown in FIG. 22B, the first pump stroke after a change in dosagerate is not delivered until the first gap time has expired. However, thepresent disclosure is not limited, and FIG. 22B can be adapted todeliver the first pump stroke at the beginning of the first gap time,such as by initializing the counter to 1 at 1424.

Referring now to FIG. 23, a flowchart depicts an example response by thecharging control circuit 754 of FIG. 16 to the Low_Battery signal.Control begins 1450, where control determines whether the Low_Batterysignal is present. If so, control transfers to 1454; otherwise, controlremains at 1450. At 1454, control sets the charging current to 0, andcontinues at 1458. At 1458, control waits for a predetermined period oftime to allow the battery voltage to recover. Control continues at 1462,where if the Low_Battery signal is still present, control transfers to1466; otherwise, control transfers to 1470.

At 1466, control waits for another period of time, which may be longerthan the period of 1458. Control then continues at 1474, where if theLow_Battery signal is still present, error handling is performed;otherwise, control transfers to 1470. The error handling may includeevaluating whether the implantable infusion device 200 is reaching itsend of life.

At 1470, control increases the charging current by an increment. Thisincrement may be a fixed increment, may be a percentage, or may be somepredetermined amount based on the previous charging current. Controlcontinues at 1478, where a step timer is set to a value and started. Thevalue of the step timer may be the same for each increasing current ormay be different for different increases in current. For example, thestep timer value may be greater for each successive increase in current.

Control continues at 1480, where if the Min_Charge signal is present,the capacitor has been charged, and therefore the capacitor chargingback-off process of FIG. 23 can stop. Otherwise, control continues at1482, where if the Low_Battery signal has once again appeared, controlreturns to 1454; otherwise, control transfers to 1486. If the step timerhas expired, control returns to 1470 so that current can once again beincreased; otherwise, control returns to 1482.

Referring now to FIG. 24, a flowchart depicts example operation of thetelemetry control circuit 774 of FIG. 16. Control begins at 1504, wherecontrol determines whether the Telemetry_On signal is present. If so,control transfers to 1508; otherwise, control remains at 1504. At 1508,control determines whether the Telemetry_Power signal is present. If so,indicating that the battery voltage is insufficient for telemetry,control transfers to 1512; otherwise, control returns to 1504. At 1512,control turns off telemetry and continues at 1516. Control remains at1516 until the Telemetry_Power signal is no longer present, indicatingthat the battery voltage has recovered. At that point, control transfersto 1520, where telemetry is turned on and control returns to 1504.

The foregoing description is merely illustrative in nature and is in noway intended to limit the disclosure, its application, or uses. Thebroad teachings of the disclosure can be implemented in a variety offorms. Therefore, while this disclosure includes particular examples,the true scope of the disclosure should not be so limited since othermodifications will become apparent upon a study of the drawings, thespecification, and the following claims. For purposes of clarity, thesame reference numbers will be used in the drawings to identify similarelements. As used herein, the phrase at least one of A, B, and C shouldbe construed to mean a logical (A or B or C), using a non-exclusivelogical OR. It should be understood that one or more steps within amethod may be executed in different order (or concurrently) withoutaltering the principles of the present disclosure.

As used herein, the term module may refer to, be part of, or include anApplication Specific Integrated Circuit (ASIC); an electronic circuit; acombinational logic circuit; a field programmable gate array (FPGA); aprocessor (shared, dedicated, or group) that executes code; othersuitable hardware components that provide the described functionality;or a combination of some or all of the above, such as in asystem-on-chip. The term module may include memory (shared, dedicated,or group) that stores code executed by the processor.

The term code, as used above, may include software, firmware, and/ormicrocode, and may refer to programs, routines, functions, classes,and/or objects. The term shared, as used above, means that some or allcode from multiple modules may be executed using a single (shared)processor. In addition, some or all code from multiple modules may bestored by a single (shared) memory. The term group, as used above, meansthat some or all code from a single module may be executed using a groupof processors. In addition, some or all code from a single module may bestored using a group of memories.

The apparatuses and methods described herein may be implemented by oneor more computer programs executed by one or more processors. Thecomputer programs include processor-executable instructions that arestored on a non-transitory tangible computer readable medium. Thecomputer programs may also include stored data. Non-limiting examples ofthe non-transitory tangible computer readable medium are nonvolatilememory, magnetic storage, and optical storage.

What is claimed is:
 1. An implantable infusion device comprising: a pump; a charge storage unit; a charging circuit configured to supply current to the charge storage unit from a battery in preparation for actuating the pump; a pump actuator circuit configured to actuate the pump using energy from the charge storage unit; and a voltage boost circuit configured to provide a boosted battery voltage generated from the battery, wherein the charging circuit is configured to supply current to the charge storage unit from the voltage boost circuit instead of directly from the battery in response to (i) a comparison of a voltage of the battery with a predetermined threshold and (ii) a comparison of a voltage of the charge storage unit with the voltage of the battery.
 2. The implantable infusion device of claim 1 wherein the pump comprises a piston and a solenoid coil, and wherein the pump actuator circuit is configured to supply a current to the solenoid coil from the charge storage unit.
 3. The implantable infusion device of claim 1 wherein the predetermined threshold is controlled in response to a firmware setting.
 4. The implantable infusion device of claim 1 wherein the comparison of a voltage of the charge storage unit with the voltage of the battery includes comparing the voltage of the charge storage unit to a predetermined percentage of the voltage of the battery.
 5. The implantable infusion device of claim 4 wherein the predetermined percentage is controlled in response to a firmware setting.
 6. The implantable infusion device of claim 1 further comprising a charge monitor circuit configured to generate a signal in response to a comparison of the voltage of the charge storage unit with a predetermined level, wherein the pump actuator circuit is enabled based on the signal.
 7. The implantable infusion device of claim 1 wherein the charging circuit is configured to decrease the current supplied to the charge storage unit in response to the voltage of the battery decreasing below a second predetermined threshold that is less than the predetermined threshold.
 8. The implantable infusion device of claim 7 wherein in response to the voltage of the battery decreasing below the second predetermined threshold, the charging circuit is configured to halt supplying current to the charge storage unit.
 9. The implantable infusion device of claim 8 wherein after halting supplying current to the charge storage unit, the charging circuit is configured to begin supplying increasing levels of current to the charge storage unit and verifying that the voltage of the battery does not decrease below the second predetermined threshold.
 10. The implantable infusion device of claim 1 further comprising: a voltage rail that is selectively connected to the battery; and a hold monitor circuit configured to monitor a voltage of the voltage rail and, in response to the voltage of the voltage rail decreasing below a first threshold, activate the voltage boost circuit.
 11. The implantable infusion device of claim 10 further comprising an isolation circuit that isolates the voltage rail from the voltage of the battery in response to a difference between the voltage of the battery and the voltage of the voltage rail.
 12. The implantable infusion device of claim 10 wherein, in response to the voltage of the voltage rail decreasing below the first threshold, the hold monitor circuit connects the boosted battery voltage to the voltage rail.
 13. The implantable infusion device of claim 12 wherein in response to the voltage of the voltage rail increasing above a second threshold that is greater than the first threshold, the hold monitor circuit disconnects the boosted battery voltage from the voltage rail and deactivates the voltage boost circuit.
 14. The implantable infusion device of claim 1 further comprising a boost control circuit configured to (i) control the voltage boost circuit and (ii) inhibit operation of the charging circuit while the voltage boost circuit is starting.
 15. The implantable infusion device of claim 14 wherein the boost control circuit is configured to inhibit operation of an alarm circuit and a telemetry circuit while the voltage boost circuit is starting.
 16. The implantable infusion device of claim 1 further comprising an electronic port finder circuit powered by the charging circuit, wherein the charging circuit is configured such that supplying current to the charge storage unit is mutually exclusive with powering the electronic port finder circuit.
 17. The implantable infusion device of claim 1 further comprising an alarm circuit configured to generate an audible alarm, wherein the alarm circuit is powered by the voltage boost circuit.
 18. The implantable infusion device of claim 17 wherein the alarm circuit disables the charging circuit while the alarm circuit is generating the audible alarm.
 19. The implantable infusion device of claim 1 further comprising a telemetry circuit that communicates with an external device, wherein operation of the telemetry circuit is inhibited while the voltage boost circuit is starting.
 20. The implantable infusion device of claim 19 wherein the telemetry circuit is configured to shut down in response to the voltage of the battery decreasing below a second predetermined threshold that is less than the predetermined threshold.
 21. A method of operating an implantable infusion device, the method comprising: supplying current from a battery to a charge storage unit in preparation for a cycle of a pump; selectively actuating the pump using energy from the charge storage unit; performing a first comparison of a voltage of the battery with a predetermined threshold; performing a second comparison of a voltage of the charge storage unit with the voltage of the battery; determining whether a boosted battery voltage is required in response to the first comparison and the second comparison; and when the boosted battery voltage is required, generating the boosted battery voltage and supplying current to the charge storage unit from the boosted battery voltage instead of directly from the battery.
 22. The method of claim 21 further comprising: generating a signal in response to a comparison of the voltage of the charge storage unit with a predetermined level; and selectively actuating the pump in response to the signal.
 23. The method of claim 21 further comprising: in response to the voltage of the battery decreasing below a second predetermined threshold that is less than the predetermined threshold, decreasing the current supplied to the charge storage unit to zero; incrementally supplying greater levels of current to the charge storage unit subsequent to decreasing the current to zero; and verifying that the voltage of the battery does not decrease below the second predetermined threshold while supplying the greater levels of current.
 24. The method of claim 21 further comprising: selectively connecting a voltage rail to the battery; monitoring a voltage of the voltage rail; in response to the voltage of the voltage rail decreasing below a first threshold, (i) generating the boosted battery voltage and (ii) connecting the boosted battery voltage to the voltage rail; and isolating the voltage rail from the voltage of the battery in response to a difference between the voltage of the battery and the voltage of the voltage rail.
 25. The method of claim 21 further comprising: while generation of the boosted battery voltage is beginning, inhibiting (i) the supplying of current to the charge storage unit and (ii) communication between the implantable infusion device and an external device; and operating an electronic port finder function mutually exclusively with supplying current to the charge storage unit.
 26. The method of claim 21 further comprising: generating an audible alarm powered by the boosted battery voltage; inhibiting generation of the audible alarm while generation of the boosted battery voltage is beginning; and inhibiting the supplying of current to the charge storage unit while the audible alarm is being generated. 